First long distrance trip after major mods

[1986F150Six]

As for the mileage, with your bigger wheels, it's probably not terrible. We'll see what the experts say... David? I'm planning on going with similar sized wheels, maybe 275/75, also on 16s, and I would be happy with 15mpg at 70mph. I have 3.08 ratio though, which makes yours even more acceptable to me.

The speed plays a big part, indeed. My happy spot seems to be 60-62 mph.

How does aerodynamic drag relate to speed?

The relationship of drag to airspeed is simple. It’s essentially the same as the “Square-Cube Law.”

Which states that if you double the speed, the drag will increase by a factor of four.

At 60 mph, think of the number 3600 [60 X 60]; now @ 75 mph, that figure jumps to 5625 [75 X 75].

At 75 mph, the drag is 56.25% greater tan @ 60 mph! This requires much more horsepower and consumes much more fuel.

And, keep in mind that the larger than stock sized wheels and or tires usually weigh much more. Plus, there is more road/tire friction as well as the amount of air being pushed ahead of the tire. This requires more power to accelerate and decelerate, so braking, ride and handling are impacted.

Another thing, as tires are larger, the truck if lifted and more air passes under the truck. This is called dirty air and causes turbulence and drag.

My stock tire size is 215/75 X 15.

[nilknarf007]

As for the mileage, with your bigger wheels, it's probably not terrible. We'll see what the experts say... David? I'm planning on going with similar sized wheels, maybe 275/75, also on 16s, and I would be happy with 15mpg at 70mph. I have 3.08 ratio though, which makes yours even more acceptable to me.

The speed plays a big part, indeed. My happy spot seems to be 60-62 mph.

How does aerodynamic drag relate to speed?

The relationship of drag to airspeed is simple. It’s essentially the same as the “Square-Cube Law.”

Which states that if you double the speed, the drag will increase by a factor of four.

At 60 mph, think of the number 3600 [60 X 60]; now @ 75 mph, that figure jumps to 5625 [75 X 75].

At 75 mph, the drag is 56.25% greater tan @ 60 mph! This requires much more horsepower and consumes much more fuel.

And, keep in mind that the larger than stock sized wheels and or tires usually weigh much more. Plus, there is more road/tire friction as well as the amount of air being pushed ahead of the tire. This requires more power to accelerate and decelerate, so braking, ride and handling are impacted.

Another thing, as tires are larger, the truck if lifted and more air passes under the truck. This is called dirty air and causes turbulence and drag.

My stock tire size is 215/75 X 15.

Sac79, your analysis is spot-on. Other than replacing the bed with a lighter (I think) wooden bed, everything I've done to the truck cosmetically has decreased it's ability to get good gas mileage. Sure, I want to get it as good as possible, but I'm not willing to sacrifice on the tires and such. I just like the way it looks.

Plus, aside from the very occasional 300 or so mile trip, it only gets about 25 miles a week put on it. So, even the worst mileage I could live with. Sometimes you'll put up with a lot for love.